The first nine petals installed at DESY were assembled on site. Engineers now plan to integrate the remaining 184 petals into this endcap. A second endcap is under construction at Nikhef, the Dutch National Institute for Subatomic Physics in Amsterdam, where another 192 petals will eventually be installed.

Why are they called petals?

The wedge-shaped units earned their botanical name from their shape, which resembles the petals of a flower fanning out from a central axis. Each petal functions as a self-contained detector slice, holding silicon sensors and support hardware in a single lightweight frame.

Strips of silicon on the petal surface detect charged particles produced in collisions. When a particle passes through the silicon, it generates a small electrical signal. Researchers read out the signal and use it to reconstruct the particle's trajectory through the detector.

How does this prepare ATLAS for HiLumi LHC?

The endcap assembly prepares ATLAS for the High-Luminosity LHC (HiLumi LHC), a future upgrade of the collider designed to deliver more particle collisions per second than the original machine. The increased collision rate will boost the volume of data available to physicists searching for rare processes, including possible new particles beyond the Standard Model of particle physics — the current best theory of fundamental particles and forces.

What comes next?

With petal installation underway, the team faces a long assembly campaign. Engineers must integrate 184 more petals into the DESY endcap and assemble a separate endcap at Nikhef. Each petal requires precise placement, connection to readout electronics, and quality testing before the completed endcaps can be shipped to CERN and lowered into the ATLAS detector.

CERN described the milestone in its 28 July update: "Petal by petal, the endcaps are beginning to bloom — bringing the ITk closer to taking its place at the heart of ATLAS to explore the physics of HiLumi LHC."

The ITk project draws on researchers and facilities from multiple countries. Petal assembly relies on specialized expertise at DESY, Nikhef, and other partner sites, with each institution contributing capabilities in silicon sensor production, electronics design, and mechanical engineering. The pace at DESY — nine petals installed in roughly two weeks — underscores the scale of work still ahead.